Response demands and the recruitment of heuristic strategies in syllogistic reasoning Carlo Reverberi, Patrice Rusconi, Eraldo Paulesu, and Paolo Cherubini University of Milan-Bicocca, Milan, Italy Two experiments investigated whether dealing with a homogeneous subset of syllogisms with time- constrained responses encouraged participants to develop and use heuristics for abstract (Experiment 1) and thematic (Experiment 2) syllogisms. An atmosphere-based heuristic accounted for most responses with both abstract and thematic syllogisms. With thematic syllogisms, a weaker effect of a belief heuristic was also observed, mainly where the correct response was inconsistent with the atmosphere of the premises. Analytic processes appear to have played little role in the time- constrained condition, whereas their involvement increased in a self-paced, unconstrained condition. From a dual-process perspective, the results further specify how task demands affect the recruitment of heuristic and analytic systems of reasoning. Because the syllogisms and experimental procedure were the same as those used in a previous neuroimaging study by Goel, Buchel, Frith, and Dolan (2000), the result also deepen our understanding of the cognitive processes investigated by that study. Keywords: Reasoning; Deduction; Syllogisms; Heuristics; Atmosphere effect; Belief bias; Mental models; Neuroimaging. Categorical syllogisms are deductive arguments composed of three quantified propositions: two premises and a conclusion. The three propositions can be either particularly or universally quantified and can be either negative or affirmative, resulting in four types of syllogistic proposition: universal affirmatives (i.e., all A are B), universal negatives (i.e., no A are B), particular affirmatives (i.e., some A are B), and particular negatives (i.e., some A are not B). The two premises share a common term (“middle term”: e.g., in “all A are B”, “no B are C”, the middle term is B). The conclusion states a relationship between the two other terms (“extreme terms”). The syllogism is valid if and only if the conclusion necessarily follows from the premises. For more than a century, syllogisms have been widely used by empirical psychologists as an arena for the study of human deduction, and we now know many details regarding how people solve them. People may sometimes recruit an array of analytic, logically consistent formal strategies, Correspondence should be addressed to Paolo Cherubini, Department of Psychology, Universita ` Milano-Bicocca, Piazza Ateneo Nuovo, 1, 20126, Milan, Italy. E-mail: paolo.cherubini@unimib.it We thank Tom Ormerod and two anonymous referees for their helpful comments. For the same reason, and for having provided the original materials, we thank Vinod Goel. # 2008 The Experimental Psychology Society 513 http://www.psypress.com/qjep DOI:10.1080/17470210801995010 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY 2009, 62 (3), 513–530 Downloaded by [Universita' Milano Bicocca] at 04:27 04 October 2011